Musnad Ahmad 32:1729

Hassan ibn Musa narrated to us, Hilal ibn Abi Dawud—meaning al-Habati, Abu Hisham—narrated to us, he said: My brother Harun ibn Abi Dawud narrated to me, he said: I came to Anas ibn Malik and said: "O Abu Hamzah, the place is far, and we like to visit you." He raised his head and said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "Any man who visits a sick person is wading into mercy. When he sits with the sick person, mercy engulfs him." He said: I said: "O Messenger of Allah, this is for the healthy person who visits the sick, but what about the sick person?" He said: "His sins are cast off from him."